Conventionally, in a charging circuit of electric automobiles such as EVs (Electric Vehicles) and PHEVs (Plug-in Hybrid Electric Vehicles) that can travel by using an electric energy accumulated in a battery as a driving source, a relay circuit for connecting or disconnecting a boost charging facility and a battery connection junction circuit at the time of charging is provided. Such a relay circuit includes a mechanical relay junction (hereinafter referred to as “relay”), and welding of the relay may be caused by the on/off performed with a high voltage and high current.
PTL 1, for example, discloses a technique for detecting welding of the relay. In the technique disclosed in PTL 1, the voltage across a positive electrode line and a negative electrode line is measured with a voltage detection circuit, and whether relays are welded is determined on the basis of results of the measurement.